Automatic frequency control system



sept. 29, 1953 l.. srAsczHovLR- ET AL AUTOMATIC FREQUENCY CONTROL SYSTEM Filed Sept. 23, 1948 INVENTORS A R ma o @n n TA A .om

of H m Patented Sept. 29, 1953 UNITED STATES PATENT OFFICE AUTOMATIC FREQUENCY CONTROL SYSTEM Leo Staschover, New York, N. Y., and Heyward A.

French, Belleville, N. J., assignors to International Standard Electric Corporation, New York, N. Y., a corporation of Delaware Application September 23, 1948, Serial No. 50,714

(Cl. Z50- 36) 6 Claims. l

The present invention relates to electrical communication systems and particularly to means for stabilizing the frequency of oscillators utilized in such systems.

In transmitters and receivers it is desirable that oscillators used therein operate at a frequency which varies little from a fixed value. To achieve this result various circuits have been proposed in which the oscillator to be controlled is adjusted under the influence of a wave derived from a very stable source and the output Wave of the oscillator. These systems are generally quite complex and in many instances of slow response resulting in relatively wide frequency variations or in other ways lacking in efficiency.

It is an object of the present invention to describe an automatic frequency control system which operates in a more efficient manner than those of the prior art.

It is another object of the present invention to provide an automatic frequency control system by which oscillators may be controlled in frequency within narrower limits than the systems of the prior art.

The above mentioned and other features and objects of this system and the manner of obtaining them will become more apparent and the invention will be best understood by reference to the following description of an embodment of the invention taken in conjunction with the accompanying drawings, wherein the single figure is a diagram illustrating the principles of the invention in a block schematic showing the means by which these principles may be applied to the automatic frequency control of well-known oscillator systems.

Referring to the figure an oscillator I, the frequency of which is to be controlled within predetermined limits, is shown feeding an oscillator utilization system 2 which may comprise either transmission or reception equipment, A standard oscillator which may be of the crystal stabilized type 3, is shown feeding a harmonic generator 4 which may be included as well known to those expert in the art. Both oscillator I and the frequency multiplied output of oscillator 3 are fed to a common mixer 5, which may be of any well known type, One will then obtain on the output line E, the products of the mixer 5 which will consist of the sum and difference of the frequencies passed to mixer 5. One of these two frequencies is selected and amplified by intermediate frequency amplifier 'I and passed to an output coupling 8. A balanced off-resonant discriminator is shown coupled to coupling 8. This off-resonant discriminator consists of two resonant circuits 9 and I0, these two resonant circuits being tuned equally above and below respectively the frequency to which intermediate frequency amplifier I is tuned. The outputs of the two resonant circuits 9 and I0 are passed to two diodes II and I2 connected in opposite polarity in such a way that the respective load resistors I3 and I4 receive oppositely poled direct current voltages. The output of this offresonant discriminator is passed to an amplifier which comprises tubes I5 and I6. An alternating current source Il is shown feeding the anodes of tubes I5 and I6 in push-pull while thel output of the off-resonant discriminator feeds the control grid of tubes I5 and I6, in parallel. The anode feed of tubes I5 'and I 6 is accomplished by means of transformer I8 and the load resistors I9 and 20. The A. C. source I1 may be of any desired frequency and in the preferred embodiment may be the power line frequency at the installation of the equipment. A portion of the alternating current voltage present on the secondary of transformer I8 is passed by line 2l to the yintermediate frequency amplifier where it is connected in such a manner as to modulate in amplitude the output yof the intermediate frequency amplifier 'I. This connection 2I may be made to any point in the intermediate frequency amplifier at which the amplifier may be modulated. As is well known such ampliers may be modulated by the control grid, suppressor grid, screen grid or at other well known points.

A pair of transformers 22 and 23 are shown connected across the load resistors I9 and 20 of tubes I 5 and I6. The secondaries of these transformers 22 and 23 are shown connected to diodes 24 and 25 which are connected in opposite polarity so that the output voltages appearing across resistors 26 and 2'I shall be in opposite polarity. rlhe voltage obtained across resistors 26 and 21 is then passed to an oscillator 4frequency' control circuit 28, which may be for example a reactance tube, servo system, or other well-known oscillator frequency control circuit. The output of the oscillator frequency control circuit 28 is then passed by line 29 to an appropriate portion of oscillator I.

The operation of the system shown in the figure may be described as follows:

The frequency of oscillator I is compared against a standard frequency obtained from oscillator 3 and harmonic generator 4 in a mixer 5. One of the frequencies obtained at the output of mixer 5 on connection 6 is amplified by an intermediate frequency amplifier 1 the output of which is amplitude modulated by an A. C. source I1. The output of the intermediate frequency amplier 1 appears across a coupling coil 8 and is passed to a discriminator comprising diodes II and I! and load kresistors I3 and 44. Across these load resistors :appears a voltage which is of a polarity indicative of the direction of frequency deviation of oscillator I and is further modulated in amplitude. This voltage is then passed in parallel to the grids of amplifiers I5 and I6 while the A. C. voltage .fromsource III `is passed to the anodes of these .tubes in push-pull. Since the discriminated voltage passed tto the grids of tubes I5 and I6 will consist sof .amplitude modulated pulses of a given polarity dependent on the oscillator frequency, and since 4the tubes I5 and I6 will conduct alternately due `to .their push-pull anode excitation, one of the tubes I5 JB -will -draw Aheavier ,plate current than the other due to -the fact that .a voltage pulse appears .at its -grid Yat the same time .that a voltage pulse is applied to its plate. This heavier current in -onevof these .tubes will result ina greater voltagepulse beingapplied to oneof therectiflers Meer 25 depending -.on whichof the tubes is conducting. The condensers `3l) .and 3.I across resistors ,26 and .21 .are .arranged to provide an integration of the voltages pulses applied to the diodes 24 and 25. The values .of .these condensers ,and their associated resistors are .chosen in such a way that the ,time constant of .the ,parallel -resistancefcapacitance combination. is long with .respect .to a period of the A. C. source ITI. Thus .since .rectiers 24 and 25 are connected in `opposite polarity itisclear that a 4polar voltage will he .developed across resistors .2B and 2J depending `upon .the direction .of frequency deviation of .the oscillator I. The voltage developed across resistors Y2li .and .21 will .consist of a steady direct .current .electromotive .force due to the integrating .action of condensers .30 and 3l. The voltage may be vthen .passed to an oscillator frequency control circuit .28 and may be used .to control the frequency of Yoscillator I by means of connection y1.9 which may be either an electrical .or mechanical connection.

While I have described above the principles of my invention .in connection .with specific apparatus it is to be clearly .understood that this description is .made only .by way of example and not as alimitation to the scope of .-my invention.

What is claimed is.:

1.1In an automatic frequency control system for an oscillation generator Shaving a frequency controlling means, the combination of a frequency discriminator, a periodic voltage source. means connected to -said source for amplitude modulating oscillations derived from the Y,gen-

fcontrolling means of erator with said periodic voltage and applying the modulated oscillations to said discriminator, a phase-amplitude detecting means coupled to said source of periodic voltage and to the output of said discriminator for deriving a control voltage having a polarity determined by the phase of the -output voltage of `said discriminator relative to said per-iodio voltage, and means for applying said control voltage to the frequency the oscillation generator.

2. A system as recited in claim 1 wherein said fdiscriminator 'includes a pair of resonant circuits tuned .on opposite sides of the frequency aof -the oscillations Aand diodes connected to said resonant circuits :in :opposed polarity whereby the output of the Iderived from said generator discriminator possesses an algebraic sign indicative of 'the direction of the frequency deviation at :said discriminator.

3. A system as recited in claim 2 wherein the phase amplitude vdetecting means comprises a pair .of .electron .tubes each including 4an anode. cathode and grid, and means for modulating the anodes in push-pull at the .same .frequency and in thesame phase .as the modulation vof said oscillations, said .discriminator outputs .being supplied Yin -parallel to -the .grids of said tubes.

4. A system as vrecited in 'claim -3 .wherein the anode circuits of said ,-.pair .of tubes are provided with means for coupling .energy into .a .second pairfof oppositely connected diodes, Athe loads of said second `pair of diodes being .connected .in series and .the voltage appearing `thereacross being applied vto `said vfrequency controlling means.

5. .A system .as recited .in claim 4 wherein the loads of said second pair of diodes comprise parallel connected .resistances .and .capacitances, the .time vconstant of .said loads being long compared .to a period :of :said periodic voltage.

6. A system as recited in claim 5 wherein the anodes vof .said pair -of .tubes are modulated in push-pull by r`means of a transformer connected to said source of periodic voltage, the secondary of said transformer being .symmetrically connected to said anodes through resistances `across which are connected said means for :coupling energy to said .second pair .of diodes.

LEO STASCHOVER. HEYWAR'D A. FRENCH.

References A(lited in the ille of this patent UNITED STATES PATENTS Number Name Date 2,065,565 Crosby Dec. 29, 1936 2,173,301 Kinross Sept. 19, 1939 2,377,326 Crosby June 5, 1945 2,377,327 Seeley June 5, 1945 2,379,689 Crosby July 3, 1945 2,445,409 Shank July V2O, 1948 

